The present invention relates to an operational amplifier, and more particularly, to a high slew-rate operational amplifier that is most suitable for driving a capacitive load, e.g., a liquid crystal panel.
In operational amplifiers, one of which is disclosed in Japanese Laid-open Patent Publication No. HEI 07-263978, a slew rate defined as a maximum rate of change of an output voltage with respect to time is an important factor in driving capacitive loads, e.g., an active matrix liquid crystal panel.
A conventional operational amplifier is shown in FIG. 7. In FIG. 7, reference symbols MP20 to MP22 represent PMOS transistors and reference symbols MN20 to MN23 represent NMOS transistors. The conventional operational amplifier includes a differential amplifier circuit 12 and an output circuit 13. The differential amplifier circuit 12 is implemented with transistors MP20, MP21, MN20, MN21 and MN22. Reference numerals 30 and 40 represent a non-inverting input terminal and an inverting input terminal, respectively. The output circuit 13 is implemented with transistors MP22 and MN23. An output node B of the differential amplifier circuit 12 is connected to a gate of the transistor MP22 of the output circuit 13 and is also connected through a phase compensation capacitor CC to an output terminal OUT of the output circuit 13. A load capacitor CO is connected between the output terminal OUT and a ground voltage VSS. The output terminal OUT is connected to the inverting input terminal 40, so that a voltage at the output terminal OUT is applied as an inverting input voltage VIN−. A non-inverting input voltage VIN+ is applied to the non-inverting input terminal 30.
In the differential amplifier circuit 12, the transistors MN20 and MN21 form an N-type differential transistor pair, and the transistors MP20 and MP21 form a current mirror acting as a load of the differential transistor pair. The transistor MN22 acts as a constant current source. The output circuit 13 is an inverter amplifier implemented with a common source transistor MP22 and a transistor MN23, which act as a driver transistor and a constant current load, respectively. A constant voltage VB1 is applied to the gate of the transistor MN22 and a constant voltage VB2 is applied to a gate of the transistor MN23.
In the conventional operational amplifier, its rising/falling waveform is gradual and its slew rate is low. These problems will be described below in more detail.
When the non-inverting input voltage VIN+ applied to the non-inverting input terminal 30 changes from a normal state, a discharging/charging slew rate (SR1) of the phase compensation capacitor CC is given by Equation 1 below.SR1=ID2/CC  [Equation 1]
where CC represents a static capacitance of the phase compensation capacitor CC, and ID2 represents a bias current flowing through the transistor MN22.
A discharging/charging slew rate (SR2) of the load capacitor CO is given by Equation. 2 below.SR2=(IO−ID2−ID3)/CO  [Equation 2]
where IO represents a current flowing through the transistor MP22, and ID3 represents a current flowing through the transistor MN23.
In order to improve the slew rate of the operational amplifier shown in FIG. 7, the discharging/charging slew rate (SR1) of the phase compensation capacitor CC and the discharging/charging slew rate (SR2) of the load capacitor CO must be improved. An important point is that a total slew rate is mainly determined by the worse (smaller) one of the two slew rates. When the load capacitor CO such as a liquid crystal driver is relatively small, the improvement of the slew rate (SR1) rather than the slew rate (SR2) is important. At this point, as can be seen from the above equations, the improvement of the slew rate (SR1) needs the current ID2 of the differential amplifier circuit 12.
However, the increase of the current ID2 causes the increase of power consumption according to the flow even in a normal state in which the current ID2 of the differential amplifier circuit 12 is considered as being equal to the non-inverting input voltage (VIN+) and the inverting input voltage (VIN−). Consequently, it is difficult to apply the conventional operational amplifier to a battery-driven mobile device, for example, a portable phone.